Superconductors are materials that can conduct electricity with little or no electrical resistance. Superconducting materials are generally characterized by a critical temperature, Tc, below which the materials are in a superconducting phase in which they can conduct an electric current without electrical resistance. Above the critical temperature, superconductors are in a normal conducting phase in which they can conduct an electric current with an electrical resistance and concomitant electrical energy loss.
The superconducting phase transition in crystalline materials results from an attractive interaction potential between pairs of electrons, known as Cooper pairs, within the crystal lattice. While electrons in a crystal lattice ordinarily experience a net repulsion for one another, it is possible for them to experience an overall net attraction for one another as a result of each electron's attractive interaction with the positive ions located at the lattice points in the crystal lattice.
Hydrogen and its isotopes can be dissolved in many metals, and can occupy the interstitial sites in the host metal crystal lattice. When hydrogen and its isotopes are dissolved in palladium (Pd), the resulting chemical compound is known as a palladium hydride, and is represented by the chemical formula PdyHx, where yH is hydrogen (1H), deuterium (2H), or tritium (3H) and x is the stoichiometric ratio of yH to Pd. As discussed below, in a palladium crystal lattice there are a maximum of 3 interstitial sites per palladium atom that can be occupied in a unit cell of the palladium crystal lattice. Consequently, palladium hydrides PdyHx can be formed having stoichiometric ratios x≦3.
Palladium naturally occurs in metallic form, and has a crystal lattice that can be classified as a face centered cubic (fcc) Bravais lattice as shown in FIG. 1. A unit cell of an fcc lattice contains 4 atoms, thus a unit cell of a palladium crystal contains 4 Pd atoms. When hydrogen ions dissolve in palladium's fcc lattice, they can occupy one of two types of interstitial sites in a unit cell of the lattice, respectively referred to as octahedral (O) and tetrahedral (T) sites. At low temperatures, dissolved hydrogen ions tend to preferentially occupy the tetrahedral interstitial sites in the palladium lattice, while at higher temperatures dissolved hydrogen ions tend to preferentially occupy the octahedral sites.
The octahedral sites, marked by an (O) in FIG. 1, are located at the center of the fcc unit cell, and along any edge of the unit cell at a point that is equidistant between two lattice vertices. Thus, there are four octahedral interstitial sites per unit cell, or one octahedral site per palladium atom in a palladium crystal lattice. The octahedral sites are so named, because each site has six nearest neighbor palladium atoms that can form the vertices of an octahedron surrounding the site.
The tetrahedral sites, marked by a (T) in FIG. 1, are located in each of the eight comers of the fcc lattice unit cell. One such corner is shown in an exploded view in FIG. 1. Thus, there are eight tetrahedral interstitial sites per unit cell, or two tetrahedral sites per palladium atom in a palladium crystal lattice. The tetrahedral sites are so named, because each site has four nearest neighbor palladium atoms that can form the vertices of a tetrahedron surrounding the site.
Palladium hydrides, PdyHx, have been known to undergo superconducting phase transitions and to exhibit the phenomenon of superconductivity since 1972. In particular, Pd1H1 is known to have a superconducting critical temperature of 9K; while Pd2H1 is known to have a superconducting critical temperature of 11K; and Pd3H0.8 is known to have a superconducting critical temperature of 4K. While the palladium hydride system PdyHx has thus been known to possess superconducting properties at or near stoichiometric ratios approaching x˜1, no one has thus far been able to explore whether the system exhibits superconducting properties at large stoichiometric ratios, characterized by x>1.
One reason for this, is that a pure palladium hydride system is unstable for stoichiometric ratios x>1. While it is possible to dissolve hydrogen ions into a palladium crystal lattice at such high concentrations, the hydrogen ions readily diffuse out of the lattice over a short period of time on the order of a few minutes. Thus, to produce a palladium hydride system with stoichiometric ratios x>1, it is necessary to develop a method to stabilize PdyHx compounds for x>1.